Fertilizers are produced by ammoniation of phosphoric acid (H.sub.3 PO.sub.4) polymerized to levels ranging from 10 to 40% poly content. Such acids are commonly termed "superphosphoric acids". Various ammonium phosphates and polyphosphates are formed. Simultaneously, the phosphate constituents are further polymerized to levels as high as 90%, more typically from about 70 to about 80%. The final product may be a fertilizer of 10-34-0, 11-37-0, and 12-40-0 (N.sub.2 -P.sub.2 O-.sub.5 K.sub.2 O) which retains only about 65 to 80% of the polymerized ammoniated phosphates. In the process, a portion of the total ammonia is reacted with a melt of superphosphoric acid followed by reaction of ammonia and water with the initial reaction product to realize a fertilizer of desired assay.
Reactions have been carried out in tubular reactors in a temperature range from about 600.degree. to about 750.degree. F. In the employment of tubular reactors, walls have been constructed of steel, stainless steel, Hastaloys, Tantalum and other corrosive resistant metals.
Despite the selection of the materials of construction scaling of the interior surface of the tubular reactors has occurred. Scale deposits contain one or more of the metals Fe, Al, and Mg complexed into ammonium pyrophosphates. Buildup of scale continues until the tubular reactor can no longer function. This requires breakdown of the reactor for cleaning.
Propane or acetylene torches are used to heat and melt part of the scale solids. Beating out the remainder of the solids with hammers is often necessary. This scores or mutilates the reactor making secondary buildup upon reassembly more rapid. Plant shutdown because of scale buildup in a tubular reactor can occur after use of 150 to 500 tons of raw material depending on the source of polyphosphoric acid used. It is believed the rate of corrosion or scale formation is a function of some balance of impurities. It appears the purer the raw material, the higher the corrosion rates.
Cooling the surface of a reactor with water, saline solutions, and the incoming phosphatic raw material has been used. This has retarded the growth of scale to a limited extent. Inevitably, plant shutdown is required to remove accumulated scale.
The present invention is directed to means to eliminate reactor scale in processes for reaction of phosphatic materials with ammonia.